Dielectric heating apparatus with automatic tunable resonant circuit



Feb. 9, 1965 c. E. M. TIBBs 3,169,230 DIELECTRIC HEATING APPARATUS WITH AUTOMATIC TUNABLE RESCNANT CIRCUIT Filed July 7, 196C m'sru Pou/Ek Wa asc/117m Mnl/HER deaf/l;

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United States Patent O l 3,169,230 DHELECTRHC HEA'HNG APPARATUS WITH-ll AUTU- MA'HC TUNABLE RESUNANT ClREUi'lf Christopher Evan Mandelli Tibbs, Earleylingland, assigner to Radio Heaters Limited, Wokingham, England, a British company Filed duly '7, i960, Ser. No. 411,307 Claims priority, application Great Britain, .lnly lli, i959, 23,809/ 59 a Claims. (Ci. 331-9) ln the design of high frequency dielectric heating apparatus, care must be given to the reduction of the radiation of electrical interference and to the restriction of such interference to the smallest possible frequency band. This problem is rendered more diiiiculty `by the fact that the resonant frequency of the output circuit of the apparatus varies considerably with different workpieces and also varies during the heating of a single workpiece owing to the change in electrical characteristics of the workpiece as its temperature rises. In free-running oscillators this resuits in considerable variation offrequency unless steps are taken to adjust the tuning. It is therefore kdesirable to provide means for retuningY the oscillator of the equipment for different workpieces and also during the heating of a single workpiece in order tov maintain the operating frequency as close as possible to a predetermined value. We have found thatit is possible to tune the oscillator automatically against a reference frequency provided by a crystal oscillator, and that this provides a satisfactory solution for some kinds of equipment.v It is not satisfactory, however, for some other kinds of 4apparatus in which the characteristics of the load change so rapidly that in spite of the automatic tuning control the oscillator would spend an appreciable amountof the total operating time outside a frequency range of i0,tl5% at 40.68 rnc/s. As an examples plastic sheet Welder might have a short welding cycle of only 1/2 of a second, during which the resonant frequency of the load might change in the ratio of ten to one.

According to the present invention the apparatus .cornprises a radio frequency oscillator of high yfrequency stabilitylincluding an output stage with a tuuabletank circuit, means which cyclically varies the resonant frequency of the tunable tank circuit about its mean frequency by a predetermined amount, and tuning means responsive to a change in output at one or bothof the extreme frequencies of thecyclic variation, resulting from a departure of the mean resonant frequency from the oscillator frequency, to adjust kthe tunable tank circuit so that itsnmean resonant frequency is restored in the direction of the ,oscillator fre- ,iquency The tuning means may include a step-by-step n motor adjusting a variable element in a directionv dependent on the departure `of `thevoutput of the tunable circuit from a limiting rvalue at one extreme frequency ofthe cyclic variation. In an alternative forrn, signals derived from thetunable circuit at the lowest and highest points of the cyclic frequency variation are rectified tand coml bined to produce a diflerencesignal which; serves to con trol the tuning means. n c j ln this way, not only is frequency stability achieved by the useof a high stability oscillator, but in addition vthe tuning of the tank circuit is maintained within predetermined limits, thereby improving the eiciency ofthe ap` Y params. Although inv most cases the high stability oscillater will be crystal-controlled, in some applications itis possible'to use a cavity-controlled oscillator, j In order that the invention ymay be better understood,

k'two embodiments will now be described with reference to theaccompanying drawings, in which: f

FIGURE l is a block circuit diagram oftheapparatus; FIGURE 2 shows the tank circuit of the power ampli-.l

. thyratronvv canconduct for a'k brief period when its anode f ICC FIGURE 3 is a diagram illustrating the operation of the tuning means.

n ln FIGURE l the crystal oscillator lil drives a power amplifier l2 the output of which is lapplied to a work circuit'ld. ln FIGURE 2 the tank circuit 16 of the power amplifier isfshown in the form of a cavity resonator having Aa cylindrical outer casing i8 provided with end plates Zliand 212 vand having an inner capacitance unit consisting of three concentric cylinders 24, 26 and 2S, cach of which has a length slightly less than that of the cylinder 18 and which are connected alternately to the end plates Ztl andi22. A section of variable capacitance is includedY in the tank circuit and is provided by `a reed 29 which is vibrated at mains frequency by a polarised armature 30, through the push-rod 3l. An adjustable coupling loop 32 provides power for the Work circuit which comprises the welding tool 33 and the output circuit tuning condenser 34, A `variable pick-up coil SSapplies an output signal from the cavity to the cathode of a diode 38, the anode of which is connected to a, capacitor dil in parallel with a resistor 42. The time constant of these components is chosen so that the radio frequency signal will be removed while the Sil-cycle waveformdue to the vibrating reed 29 will kbe substantially unaffected. The negative voltage across the capacitor itl is applied in parallel to the input circuits of two thyratrons 44 land to which are biased byY a common battery d8. The anodes of the two thyratrons are coupled-through coilsV 50 and 52,`respectively, to the v secondary winding of a mains'transforrner 54, the centre tappingrof the secondarywinding being earthed. The coils Sil and 52 are the energising coils for two step-bystep motors 56 and 58 which serve to adjust a variable pulling dueto the changes inthe workA circuit. The

maximum voltagefis vdeveloped across the tank circuit when the resonant frequency ofthe latter is `equal to the frequency of the crystal which controls thefrequency applied tofthe tank circuit. lWhen they mean resonant' frequency of the tankv circuit has this value the effect of the vibrating reedwill be to swing the resonant frequency between the limits f1 and f2 of FIGURE El,` and consequently at theextreme values of ther cyclicfrequency variation, 4equal voltagesal and b1 will`be applied to the rectiter. When the mean resonant frequency of the f tank circuit isv toolow and has, for examplepthe value f3, the extremes of the cyclic frequency variation will be lthe frequencies f4 and f5 corresponding to voltages a2 and b2." Ina similarjway when the mean resonant frequency hasa value f6 which is greater than the' ,oscillator fre'- quency rthe extremes of the cyclic frequency variation l will beffq [and f8' corresponding to voltages a3 and b3. The j phase ofthe alternating voltageswhich are applied :to the anodes of the. two thyratrons 44 and dois such that one'anode willv haveits' peak positive'voltage when` the vibrating reed 29 is infone extreme position and the other anode will have its peakpositiveivoltage when the vibratingreed is in its other extreme position. The value of" i the bias applied to the thyratrons 'is suchthat each n receives ythe peak positivev voltage if the negative Voltage applied pto' then .thyratron gridffrom the4 capacitor 40 r,bvecornes less vnegativethan a-predeterminedvalue- .This

i predetermined-value is represented in FIGURE. @by the f line V1. "Thus-whenthemean resonant fretplencyL of the cavity tank circuit is substantially correct both voltages a1 and b1 are below the predetermined value V1 and the thyratrons do not conduct. When the mean resonant frequency has the value f3 the thyratron which receives the peak anode voltage when the capacitor 40 receives the voltage a2 conducts, but the other thyratron, which receives the voltage bz when its anode receives the peak positive voltage, does not conduct. The conducting thyratron will be extinguished as soon as the anode voltage falls below .a given value. The pulse of current which ows through the corresponding coil 50 or 52 will cause the step-by-step motor to adjust the tuning capacitor 59 through one step in a direction such that the mean resonant frequency of the work circuit is moved towards the oscillator frequency. In a similar manner when the mean resonant frequency has a value f6, the other thyratron will conduct since it will receive `a voltage b3 which is above the predetermined value V1 when its anode has the peak positive value. As a result the other step-by-step motor will be energised and will move the variable capacitor 59 in the opposite direction.

1n this way, although the resonant frequency of the tank circuit is continuously varying between the limits f1 and f2, it does not vary outside predetermined limits which are established by the selected voltage V1. As a result the tuning of the tank circuit is always within a ertain frequency range centred on the crystal frequency and as a result the heat dissipation at the anode of the power amplifier valve is never excessive.

As an example, in `apparatus having a tank circuit for which the working Q is 250, a suitable frequency swing would be from 0.01% to 0.4% of the operating frequency of .the equipment.

A reactance valve can also be used for the frequency Wobbulation of the tank circuit tuning. In this case it is arranged that the signal which causes the reactance valve to vary the frequency also operates the grid circuit of one rectifier valve at one extreme frequency and that ofthe other rectifier valve at the other extreme frequency.

The crystal oscillator and the power output valve are enclosed within a screened compartment to reduce harmanic radiation, andthe leads into the screened compartment are filtered to reduce the escape of harmonic radiation through theseleads. `The tank output circuit is also within `a screened enclosure, which may be combined with that enveloping the output valve. It is also desirable to include harmonic filters in the valve anode lead and also in the output cad, and the output may be inductively coupled to a separately tuned work circuit with a Faraday screen interposed between the output circuit and the tuned work circuit.

We have found that a reduction of the harmonic radiation from the equipment can be obtained by using a pushpullcavity power amplifier stage, as described in our copending British application No. 21564/60.V in this circuit, thepush-pull triodes are driven from the ends of a push-pull input transformer, and their anodes are connected through coupling capacitors with adjacent cylinders of `the central capacitance unit within the cavity tank circuit, the adjacent cylinders constituting opposite electrodes of the central capacitance unit. The cathode-s of the two triodes are connected to opposite end plates of CII the cavity, and an output pickup loop emerges Vthrough the cylindricalvside wall of the latter. The coefficient of inductive coupling between the tank circuit and the work circuit preferably has a low value which does not exceed .the critical coupling by a factor of more than 20, and

preferably not by a factor of more than 5.

I claim: 1. Dielectric heating apparatus comprising:l a radio frequency oscillator stage of ,high frequency stability; anA f output stage driven` from said oscillator stage and having a tunable tank circuit; means which cyclically varies the resonant frequency of said tunable tank circuit about its mean frequency by a predetermined amount; and tuning means responsive to changes in the output levels of the tank circuit at the two extreme frequencies of said cyclic variation, resulting lfrom the departure of the mean resonant frequency from the oscillator frequency, to adjust said tunable tank circuit so that its mean resonant frequency is restored in the direction of the oscillator frequency 2. Apparatus according to claim l, in which said tank circuit includes a tunable element in the form of a variable capacitor including a vibratable reed.

3. Apparatus according to claim 1, in which the tunable circuit is adjusted by a step-by-step motor controlled by a relay device operated at one extreme frequency of the frequency sweep.

4. Apparatus according to claim 3, including two step- `by-step motors for adjusting the tunable circuit in opposite directions, the energising coils of the step-by-step motors being pulsed by means of thyratrons operative at opposite ends of the frequency sweep.

5. Apparatus according to claim 1, in which signals derived from the tunable circuit at the lowest and highest points of the frequency variation are combined to provide a difference signal which serves to control the tuning means.

6. Apparatus according to claim l, in which the tunable circuit incorporates as a tuning element a vibrating reed.

` 7. Dielectric heating apparatus comprising: a radio frequency oscillator stage of high frequency stability; an output stage driven from said oscillatorstage; a tank circuit in said output stage, including two tunable elements therein; means which continuously and cyclically adjusts one of said tunable elements to vary the resonant frequency of the tank circuit about its mean frequency by a predetermined amount; further means for adjusting the other of said tunable elements to vary the mean frequency of said cyclic variation of frequency of the tank circuit; and tuning means responsive to changes in the output levels of said tank circuit at the two extreme frequencies of the cyclic variation, resulting from a departure of the mean resonant frequency from the oscillator frequency, to operate said further adiusting means to restore the -mean resonant frequency of the tank circuit in the direction of the oscillator frequency.

8. Dielectric heating apparatus comprising: a radio frequency oscillator stage of high frequency stability; an output stage driven from said oscillator stage; a cavity resonator in the anode circuit of said output stage; two variable elements in said cavity resonator; means which continuously and cyclically adjusts one Aof said variable elements to vary the resonant frequency of said cavity resonator about its mean frequency by a predetermined' amount; further means for adjusting the other of said variable elements to vary the mean frequency of said cyclic variation of frequency of said cavity resonator; pick-up means coupled to said resonator for obtaining a cyclically varying control voltage of which the peak values in each cycle occur at the extremes of said cyclic frequency variation; and tuning means responsive to said control voltage only when said peak values exceed a predetermined amount to operate said further adjusting means to restore the mean resonant frequency of said cavity resonator in the direction of the oscillator frequency.

'References Cited in the tile of this patent UNETED STATES PAFENTSr Gensel Aug. 14, 1951 

1. DIELECTRIC HEATING APPARATUS COMPRISING: A RADIO FREQUENCY OSCILLATOR STAGE OF HIGH FREQUENCY STABILITY; AN CUTPUT STAGE DRIVEN FROM SAID OSCILLATOR STAGE AND HAVING A TUNABLE TANK CIRCUIT; MEANS WHICH CYCLICALLY VARIES THE RESONANT FREQUENCY OF SAID TUNABLE TANK CIRCUIT ABOUT ITS MEAN FREQUENCY BY A PREDETERMINED AMOUNT; AND TUNING MEANS RESPONSIVE TO CHANGES IN THE OUTPUT LEVELS OF THE TANK CIRCUIT AT THE TWO EXTREME FREQUENCIES OF SAID CYCLIC VARIATION, RESULTING FROM THE DEPARTURE OF THE MEAN RESONANT FREQUENCY FROM THE OSCILLATOR FREQUENCY, TO ADJUST SAID TUNABLE TANK CIRCUIT SO THAT ITS MEANS RESONANT FREQUENCY IS RESTORED IN THE DIRECTION OF THE OSCILLATOR FREQUENCY. 